1. Field of the Invention
The present invention relates to a mark detecting apparatus applicable to a microfilm retrieving apparatus for feeding an elongated microfilm to retrieve a given image frame of such a microfilm. More particularly, it relates to a mark detecting apparatus having a plurality of light receiving means.
2. Related Background Art
In FIG. 12, a microfilm retrieving apparatus having a conventional mark detecting apparatus is shown. A microfilm 100 with marks provided corresponding to image frames is fed by the rotation of winding reel 101 and rewinding reel 102 to pass between the glasses 103 and 104 arranged in parallel. Below the glass 104, there are provided a light source 105 and a condenser lens 106, while above the glass 103, a projection lens 107, a mirror 108, and a screen 109.
Also, with the upper surface of the glass 103, the light receiving planes of a plurality of optical fibers 110 are arranged closely in contact. At the other ends of the optical fibers 110, the photoelectric conversion elements are arranged to output electric signals corresponding to the luminous energy drawn by the optical fibers 110. Each of the photoelectric conversion elements 111 is connected to an amplifying circuit 112. Further, the CPU circuit 115 serving as control means is connected to the discriminating circuit 114 and the binarizing circuit 113. The optical fibers 110 and photoelectric conversion elements 111 constitute light receiving means.
To the amplifying circuit 112, a binarizing circuit 113 is connected. Further, a CPU circuit 115 serving as control means is connected to a discriminating circuit 114 and a binarizing circuit 113. The CPU circuit 115 sets the reference values for the binarizing circuit 113.
The aforesaid winding reel 101 and rewinding reel 102 are controlled by the driving of motors 116 and 117. The motors 116 and 117 are driven by a carrier control circuit 118 connected to the CPU circuit 115. In this respect, a reference numeral 119 designates a key board connected to the CPU circuit 115.
With the structure described above, the motor 116 or 117 is driven by operating the key board 119 to feed the microfilm 100 in a given direction. At the same time, the microfilm 100 is irradiated by a light source 105. Then, the transmitting rays of light are drawn by the optical fibers 110 so that electric signals are output from the photoelectric conversion elements 111. After being amplified by the amplifying circuit 112, the electric signals are binarized by the binarizing circuit 113 in relation to the reference values supplied by the CPU circuit 115. Thus, the discriminating circuit 114 discriminates the presence of marks in accordance with the binarized signals and transmits the results to the CPU circuit 115.
When a desired mark is detected, the feeding of the microfilm 100 is suspended through the CPU circuit 115. Hence, the transmitting rays of light from the image frame illuminated by the light source 105 are projected to the screen 109 through the projection lens 107 and mirror 108.
Now, there are the following methods for setting the reference values:
(1) Using a detecting apparatus, that is, variable resistors and others provided for light receiving means, a service person makes a special adjustment. Also, using the data which are prepared by the use of dedicated adjustment tools at a factory or some other location, reference values are established. PA0 (2) Film detecting means different from the light receiving means for mark detection, a reflective sensor or the like, for example, is provided for the microfilm feeding pass. Then, the output signals corresponding to the incident light to the reflective sensor are obtained when there is no microfilm. Utilizing such output signals, the reference values for binarization are defined. PA0 (3) The transmitted rays of light from the portion of a microfilm where no marks are provided, that is, the base portion of the microfilm, are received by light receiving means, and reference values are established on the basis of the luminous energy of light thus received.
However, the following problems are encountered in the above-mentioned conventional examples:
As regards the methods (1), (2), and (3), it is impossible to discriminate the case where the output values are lowered due to the presence of dust or marks between the light receiving means and the light source from the case where the luminous energy of the light source itself varies. As a result, when there are changes in a light source in service, necessitating modifications of the binarized reference data, it is impossible to detect the variations of the luminous energy accurately for the purpose.
When the base density of the film to be used for retrieval is high, the binarized reference data should be optimized in accordance with such a base density. In this case, too, the same problem as above is encountered. Consequently, if the film exists in the feeding pass, it is impossible to modify the binarized reference data. Also, the method (3) results not only in the increased number of light receiving means, leading to an increased cost, but also in the incapability of obtaining any correct outputs when the base density is significantly changed. As a result, there is a possibility that an erroneous detection takes place in either case.